As the world's population expands and economic activity increases, there are ever increasing signs that increasing atmospheric concentrations of carbon dioxide are warming the earth causing climate change. While the eventual depletion of the world's oil and fossil fuel energy sources will inevitably require other economic energy sources to be found, the more noticeable signs of global warming have increased pressures for global energy systems to move away from carbon rich fuels whose combustion produces carbon monoxide and carbon dioxide gases.
Hydrogen energy is attracting a great deal of interest and is expected to eventually be a replacement for petroleum based fuels. However, there are still several technical issues and barriers that must be overcome before hydrogen can be adopted as a practical fuel, the main obstacle being the development of a viable hydrogen storage system. While hydrogen can be stored as a compressed gas or a liquid, the former occupies a large volume and the latter is energy intensive to produce, reducing any environmental benefits. In addition, both gaseous and liquid hydrogen are potentially dangerous should the pressure storage vessels be ruptured.
A safer, more compact method of hydrogen storage is to store it within solid materials. When infiltrated with hydrogen at relatively low pressures, metals and inter-metallic compounds can absorb large quantities of hydrogen in a safe, solid form. The stored hydrogen can be released when required by simply heating the alloy. Storage of hydrogen as a solid hydride can provide a greater weight percentage storage than compressed gas. However a desirable hydrogen storage material must have a high storage capacity relative to the weight of the material, a suitable desorption temperature, good kinetics, good reversibility and be of a relatively low cost.
Pure magnesium has sufficient theoretical hydrogen carrying capacity at 7.6 wt %. However the resulting hydride is too stable and the temperature must be increased to 278° C. for the hydrogen to be released. This desorption temperature makes such materials economically unattractive. A lower desorption temperature is desirable to not only reduce the amount of energy required to release the hydrogen but to enable the efficient utilisation of exhaust heat from vehicles to release the hydrogen. Compared to pure magnesium, the compound Mg2Ni has a reduced hydrogen storage capacity of 3.6 wt % but, importantly, the temperature required for hydrogen release is decreased to less than that of pure magnesium. The mechanism of hydrogen storage is believed to involve the formation of (solid) hydride particles, i.e. MgH2 and Mg2NiH4 in the microstructure.
Recently, thixotropic casting techniques followed by partial remelting and quenching have been used [Y.-J. Kim, T.-W. Hong: Materials Transactions 43 (2002) 1741-1747] to produce hypoeutectic Mg—Ni alloys consisting of magnesium rich dendrides surrounded by refined Mg—Mg2Ni eutectic. These alloys absorb large amounts of hydrogen, similar to pure magnesium and display only a single hydrogen absorption plateau in the pressure-composition-temperature (PCT) curve, i.e. not separate plateaus for each phase. It is believed that the nickel and/or Mg2Ni phase acts as a catalyst, improving the kinetics of hydrogen transfer into the magnesium rich solid phases via MgH2 formation.
This realisation has encouraged research [See review by S. Orimo and H. Fuji, Applied Physics A 72 (2001) 167-186] using nano technology and powder metallurgy techniques to produce materials with large internal interfacial areas. These techniques are attractive because they result in large interface areas and they introduce crystallographic defects such as dislocations and twins, which could distribute potential catalysts throughout the microstructure, enabling them to have a widespread influence on the kinetics of the reaction. Unfortunately nano-scale powder metallurgy techniques offer limited control over the crystallographic structure of the phases (ie. interfaces, twins etc), the powder would be highly explosive and would be prohibitively expensive for large-scale mass production of commercial hydrogen storage components. None of the research reported to date considers methods by which higher performance hydrogen storage components can be produced using lower cost processes more applicable to mass production.
It is an object of the present invention to provide a castable MgNi alloy with improved hydrogen storage capabilities.
Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction.